handle PDF thumbnails

[antonkorotkov]

Looks like thumbnails are being generated for PDFs.

Example:

But they are not being deleted after syncing in Ephemeral mode. I think this is something relatively new. And it is not related to the background processing.

[planv]

Current issue with removing thumbnails in Ephemeral mode fixed.

Also I noticed that we don't create thumbnails for PDF in Stateless mode, it is due to using WP_Image_Editor_GD (for StreamWrapper that doesn't save file locally) instead 'WP_Image_Editor_Imagick' (need for generating thumbnails for PDF - https://make.wordpress.org/core/2016/11/15/enhanced-pdf-support-4-7/ ).
@ewsopp Should we add notice about that?

[ewsopp]

I think so, but when would we display it? and what would it say?

[planv]

@ewsopp

I guess here

Store and serve media files with Google Cloud Storage only. Media files are not stored locally, but local storage is used temporarily for processing and is required for certain compatibilities, generating thumbnails for PDF documents.